1. Field of the Invention
The present invention relates to a steering wheel for steering a vehicle, and more particularly to a steering wheel incorporating a core metal of an annular ring portion, which is held when steering is performed, structured such that a hard portion made of hard synthetic resin and a soft portion made of soft synthetic resin softer than the hard portion are disposed around the core metal such that the hard portion and the soft portion are disposed in the planar circumferential direction of the ring portion.
2. Description of the Related Art
Hitherto, a portion of steering wheels has a structure that a hard portion made of hard synthetic resin or a woody material and a soft portion made of soft synthetic resin softer than the hard portion are disposed around a core metal of an annular-ring portion which is held when steering is performed such that the hard portion and the soft portion are disposed in the planar circumferential direction of the ring portion.
Hitherto, a portion of wood steering wheels each having a woodgrain finish has a structure that woody members are partially bonded to the outer surface of the ring portion. Another portion has a structure that a woody member is provided for the overall circumference of the ring portion by molding.
The conventional steering wheel of the foregoing type is structured such that the hard portion made of the hard synthetic resin or the woody material is partially bonded to the core metal of the ring portion in the planar circumferential direction. When the foregoing type steering wheel is arranged such that the hard portion formed in the planar circumferential direction of the ring portion by a bonding operation and the soft portion formed to cover the core metal of the ring portion by a molding operation, such as injection molding are provided, the following problem arises.
That is, when the steering wheel of the foregoing type is manufactured, the soft portion is molded before the hard portion is bonded to prevent formation of burrs of the soft portion on the surface of the hard portion. When the soft portion is molded, a dimensional error of the core metal sometimes inhibits the core metal of the ring portion to be disposed in the central portion of the cross section of the cavity if the core metal of the ring portion is introduced into the mold. If the soft portion is molded and the hard portion is then bonded to the core metal of the ring portion in a state where the deviation of the foregoing type occurs, unnecessary stepped portion 204 occurs between the hard portion 202 and the soft portion 203, as shown in FIG. 28. The reason for this will now be described. For example, the upper and lower hard portions 202 bonded to the core 201 of the ring portion can be disposed without any deviation from the core metal 201, as shown in FIG. 29. Since the soft portion 203 covering the core metal 201 is deviated from the core metal 201, a stepped portion 204 is formed between the hard portion 202 and the soft portion 203. The foregoing stepped portion 204 deteriorates the quality of the design of the appearance of the ring portion R. As a result, there arises a problem in that a defective steering wheel is produced.
The foregoing steering wheel is manufactured by bonding the hard portion to a portion around the core metal of the ring portion. A portion of the steering wheels has a structure that both of the hard portion and the soft portion are formed by injection molding. A steering wheel of the foregoing type and the conventional problem will now be described.
The steering wheel of the foregoing type has a structure that a hard portion made of hard synthetic resin and a soft portion made of soft synthetic resin softer than the hard portion are disposed around the core metal of the ring portion constituted by a steel pipe such that the hard portion and the soft portion run in the planar circumferential direction of the ring portion. The hard portion is disposed at each of the front and rear portions of the steering wheel. Each of the portions of the hard portion is made of hard synthetic resin, such as foamed epoxy resin which covers the core metal of the ring portion. On the other hand, the soft portion is disposed in each of the right and left portions of the steering wheel, the soft portion being made of foamed urethane which covers the portion around the core metal of the ring portion.
Note that the surface of the hard portion has a thin cover formed into a woodgrain finish. The surface of the soft portion has a leather cover.
The foregoing steering wheel is manufactured such that the core metal of the steering wheel is manufactured. Then, the core metal of the steering wheel and the cover are introduced into a mold for molding the hard portion. The front and rear hard portions are collectively injection-molded. Then, the right and left soft portions are collectively. injection-molded. Then, the leather member is wound around the soft portion.
If the steering wheel of the type having the hard and soft portions disposed in the planar circumferential direction of the ring portion is, for a long time, allowed to stand in the cabin of a vehicle, the temperature of which is raised, a crack sometimes occurs in the hard portion. The reason for this lies in that the coefficient of thermal expansion of the core metal of the ring portion made of the steel pipe and that of the hard portion are considerably different from each other (the core metal of the ring portion has a small coefficient of thermal expansion and the hard portion has a large coefficient of thermal expansion). Also the soft portion and the core metal of the ring portion are greatly different from each other in the coefficient of thermal expansion. Since the soft portion made of the soft material is able to absorb the difference in the coefficient of thermal expansion by elastic deformation and the like, cracks cannot easily occur.
When the hard portion is made of a low-cost polypropylene or ABS resin, the coefficient of thermal expansion of the foregoing low-cost material is made to be furthermore different as compared with the foamed epoxy resin. Therefore, there arises a problem in that the crack easily occurs.
In general, the foregoing steering wheel is structured such that the volume of the front portion of the front and rear portions which are collectively formed by injection molding is made to be larger than the volume of the rear portion.
Therefore, the molding operation is performed such that injection of the material for molding into the cavity for molding the rear hard portion is completed prior to completion of injection of the material for molding into the cavity for molding the front hard portion.
As a result, there arises a problem in that a large burr is formed adjacent to the rear hard portion as compared with the portion adjacent to the front hard portion. Therefore, there arises a problem in that an excessively large labor is required to remove the burr.
When the hard portion of the ring portion is formed, the conventional steering wheel has been molded such that one of gates arranged to inject the material for molding into the cavity of the mold and disposed at the end of the hard portion in the planar circumferential direction of the ring portion is employed, the one of the gates being disposed at a vertical-directional intermediate position of the steering wheel on the cross section of the ring portion.
The hard portion of the ring portion of the foregoing steering wheel is sometimes formed such that the gate for injecting the material for molding into the cavity of the mold is, as one of gates disposed at the ends in the planar circumferential direction of the ring portion, disposed at an intermediate position in the vertical direction of the steering wheel on the cross section of the ring portion. In the foregoing case, there arises a problem in that a weld mark is formed on the upper surface.
The core metal of the ring portion is sometimes formed by die-casting as a substitute for the steel pipe to form an inverted U-shape cross section. When the gate for molding is disposed at the intermediate position in the vertical direction of the steering wheel of the cross section of the ring portion in the foregoing case, the material for molding easily flows along the inner surface of the core metal of the ring portion. Thus, the material for molding is injected into the inner surface portion of the core metal of the ring portion. Then, the material for molding is injected into the upper surface portion of the core metal of the ring portion from the lower end portions of the two side wall portions of the cross section of the core metal of the ring portion. Therefore, a weld mark is furthermore easily formed in the upper surface portion of the core metal of the ring portion.
If the weld mark is formed in the upper surface portion, an individual coated film or the like must be formed afterwards. Since the upper surface portion is a portion which is directly observed when the steering wheel is used, the quality of the appearance of the ring portion of the steering wheel deteriorates if the foregoing coated film or the like is not provided. When a glossy coated film having a small thickness is formed, the weld mark is sometimes undesirably highlighted.
A support portion for supporting the inner surface of the leather portion bonded to the surface of the soft portion is formed at an end of the hard portion of the steering wheel of the foregoing type. Moreover, woodgrain grooves into which the ends of the leather portion are inserted are formed in the base portions of the support portion.
The leather portions are sewed together in the inner surface portion of the ring portion and, as well as bonded to the soft portion. Moreover, the inner surface of each end portion is supported by the outer surface of the support portion of the hard portion. In addition, the ends are inserted into the woodgrain grooves so as to be secured to the hard portion.
The reason why the major portion of the leather portion is provided for the soft portion lies in that a satisfactory touch can be realized in the foregoing case when the foregoing portions are held by the hands.
The reason why the ends of the leather portions are supported by the hard portions lies in that sags of the ends of the leather portion can be prevented in the foregoing case.
When the conventional steering wheel around which the leather is wound is manufactured, the core metal of the steering wheel is manufactured. Then, the core metal of the steering wheel and the coating member are introduced into the mold for molding the hard portion. Thus, the hard portion is molded. Then, the soft portion is molded, and then the leather is wound around the soft portion so as to be sewed and bonded to the soft portion. Then, the ends of the leather are supported by the outer surface of the support portion of the hard portion. In addition, the ends of the leather are inserted into the woodgrain groove.
The structure that the ends of the leather are inserted into the woodgrain groove to secure the leather to the hard portion, however, encounters a problem in that the end of the leather is separated from the woodgrain groove when the leather has been contracted owing to use of the steering wheel for a long time. Thus, there is apprehension that the appearance of the ring portion deteriorates.